1. Field of the Invention
This invention relates generally to a method for encoding tracking tone modulation for application in magnetic recording. More particularly, this invention relates to an improved method and apparatus for encoding a 24/25 modulation code adopted by an international standard committee of consumer digital video cassette recorder (VCR) whereby faster encoding speed and lower integrated circuit (IC) gate count can be achieved to improve operational flexibility while achieving lower manufacture costs.
2. Description of the Prior Art
For the design and manufacture of modern digital video cassette recorders (VCRs), two of the most important design considerations for auto-track following (ATF) are the encoding speed and the hardware requirement. In a conventional system, high integrated circuit (IC) gate count is required in order to achieve high speed ATF encoding which is necessary to achieve higher level of performance. However, in order to achieve a second set of design goals of reducing production cost while increasing reliability, the gate count and the number of ICs should be kept as low as possible to simplify the hardware configuration. Therefore, when conventional encoding techniques are applied, these design goals impose conflicting gate count requirements. An encoding apparatus with simple design which is formed with low IC gate count while capable of providing high encoding speed is not available.
In 1993, under the sponsorship of Matsushita, Philips, Sony, and Thompson, a global conference was held which was participated by most of the major consumer electronic manufacturers. The purpose of that conference was to evaluate and determine a common standard for digital VCRs, particularly for VCRs manufactured for the consumer markets. A draft standard was produced based on a proposal initiated by Matsushita and Sony. The draft standard covers the current standards employed in television broadcasting such as NTSC, PAL, and also the standards to be employed in future high definition TVs. A 24/25 modulation code was adopted in this draft standard. This modulation code has high encoding efficiency. It has additional advantage in that the DC components are eliminated from the efficiency spectrum. Furthermore, at the frequency of either 1/90 Tb or 1/60 Tb (where Tb represents bit per transfer period), a peak or notch can be generated to serve as the pilot tracking tone. The space requirement for storing the auto-track following (ATF) signals can thus be reduced in performing the helical scan.
The technique of helical scan has long been applied to analog VCRs. Recently, helical scan has also been used in digital audio tapes, 8 mm data cartridges, and D1, D2, and D3 video recording systems. For magnetic recording, helical scan technique is recognized as the most efficient method; however, complicated mechanical structure and control techniques must be employed. The operation of the auto-tracking function (ATF) is the most difficult portion in controlling the recording system. In reading the data stored in the magnetic storage media, in addition to reading and restoring the original stored data on the storage media, a reference signal for auto-tracking must also be generated to assure that the magnetic heads are precisely tracking the recording media which are typically formed as magnetic strips. FIG. 1 shows the operation of such an ATF function. There are three recording strips on the storage media, i.e., F0, F1, and F2 tracks. Each of these strips contains data for generating track following power spectrum as shown in FIG. 2 wherein there are peaks and notches at specific frequencies, e.g., freq1 and freq2. Three magnetic heads, i.e., heads 1, 2, and 3 are also shown in FIG. 1 where the widths of these heads are slightly wider than the tracks. The magnetic head 2 is positioned over the mid-portion of the F0 track and extends over to F1 and F2 tracks. The power spectrum generated from the signals detected by head 2 is used to cancel out each other thus generating a calibration power difference of a value of zero between freq1 and freq2. While the power difference generated from signals detected by head 1 and head 0 is used to generate a calibration power difference which has either a positive or negative value respectively. Depending on the signs of the calibration power difference, the head is controlled and calibrated to generate a zero calibration power spectrum to maintain the head at a position right on top of the middle portion of strip F0 as shown for head 2. As the traditional modulation codes cannot be used to generate the power spectrum as shown in FIG. 2, a separate zone on a video tape must specifically be assigned for ATF purpose as shown in FIG. 3. The total usable areas for recording video data are reduced due to this requirement of separately assigned ATF zones. Furthermore, since only part of the magnetic recording media contains ATF data, more advanced track following features, such as dynamic head tracking cannot be performed.
The 24/25 modulation code which is adopted in the video cassette recorder (VCR) draft standard provides a highly efficient encoding method to overcome the above limitations. The magnetic strips used for recording video data can also be encoded to generate ATF signals. A global ATF feature is achievable while the recording space is expanded because a separate Zone for ATF calibration, used in a traditional track following technique, is no longer required. FIG. 4 shows a word {D.sub.k .vertline.k=0, 1, 2, . . . }, includes three bytes, wherein every bit is defined as {d.sub.k,i .vertline.i=23, 22, 21, . . . 0}. The 24/25 modulation code is implemented by adding a bit e.sub.k for each word D.sub.k. The modulated output code can be represented as {o.sub.n =c.sub.k,i .vertline.n=25*k+24-i} and c.sub.k,i is further defined as: ##EQU1## Equation (1) is generally referred to a an interleaved NRZI or abbreviated as INRZI. In the standard for 24/25 modulation encoding, certain specific rules of run length limit and power spectrum criterion are applied to determine whether e.sub.k is a bit of either zero or one.
In order to generate generate peaks and notches at freq1 and freq2, e.g., f.sub.p /60 and f.sub.p /90, a spectrum peak and notch power monitoring (SPNPM) scheme is employed in a conventional modulation encoder. The SPNPM scheme is based on the theory that a peak at freq1 and freq2 will be generated when the e.sub.k is selected such that the power spectrum P.sub.k is maximized where P.sub.k is defined as below: ##EQU2## Based on the SPNPM scheme, a notch is generated at freq1 and freq2 by selecting e.sub.k to minimize P.sub.k as defined in Equation (2).
In Equation (2), the power spectrum P.sub.k has a real part pr.sub.k and an imaginary part pi.sub.k which can be calculated from the real part sr.sub.k and the imaginary part si.sub.k as the followings: ##EQU3##
Based on the above equations, assuming that the added bit e.sub.k is determined to be e, the values of the power spectrum can be calculated as:
i) The DC component of the power spectrum, i.e., dcp.sub.k.vertline.e, can be calculated as P.sub.k in Equation (2) by substituting e.sub.k with e and setting fp=0.
ii) The power spectrum of the real components of a peak, i.e., ppr.sub.k.vertline.e, can be calculated as pr.sub.k in Equation (4) by substituting e.sub.k with e and setting fp=freq1 for track F1 and fp=freq2 for F2.
iii) The power spectrum of the imaginary components of a peak, i.e., ppi.sub.k.vertline.e, can be calculated as pi.sub.k in Equation (4) by substituting e.sub.k with e and setting fp=freq1 for track F1 and fp=freq2 for track F2.
iv) The power spectrum of the notch, i.e., np.sub.k.vertline.e, can be calculated as p.sub.k in Equation (2) by substituting e.sub.k with e and setting fp=freq1 for track F1 and fp=freq2 for track F2.
v) The power spectrum of notch, i.e., np.sub.k.vertline..sup.x.sub.e, at freqX, can be calculated as p.sub.k in Equation (2) by substituting e.sub.k with e and setting fp=freqx for track F0.
In order to carry out above computations by the use of logic circuits in an integrated circuit chip, a very complex circuit design has to be implemented requiring a very high gate count. In a modulation encoding apparatus to manage the SPNPM encoding computations, the complexity of the encoding apparatus is increased due to the facts that: a) sine and cosine functions are used, b) square and division calculations are involved, and c) five layers of logic decision making processes are performed. Thus, a highly complex IC apparatus with large gate count is necessary to perform the 24/25 modulation encoding for carrying out ATF functions.
Therefore, a need still exists in the art of automatic track following (ATF) by applying the 24/25 modulation encoding to provide a new and improved encoding apparatus with reduced gate count by implementing a simplified encoding scheme to overcome the difficulties in the prior art Specifically, the simplified encoding procedure must be able to reduce the complexity of the encoding processes caused by required computations of sine and cosine functions, the square and division operations, and the five layers of logic decisions required in the conventional SPNPM methods.